margin:## 0 0 ##px; is just the shorthand for margin properties. it allows you to put in a top margin, bottom margin, etc., without writing out
margin-top:##px; margin-bottom:##px;. the dimensions go counter clockwise:
CODE
margin:TOP RIGHT BOTTOM LEFT;
you only need the "px" or "%" or "em" after the last property.
you can also use shorthand for top/bottom, right/left combinations:
CODE
margin:## ##px;
where the first ## is the dimension for both the top and bottom margins, and the second ## is for both right and left.
as for the code you're using, that will unfortunately only work in firefox.